Attenuators



May 28, 1963 c. EISAMAN ET AL 3,091,741

ATTENUATORS 3 Sheets-Sheet l Filed April 18, 1957 new o o O 3 2 COILCURRENT- MA.

INVENTOR. LEO C. EISAMAN BY SEYMOUR SCHIFF ATTORNEY May 28, 1963 I.. c.EIsAMAN ETAL 3,091,741

ATTENUATORS Filed April 18, 1957 5 Sheets-Sheet 2 OSC FREQUENCY ,4

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DEVICE (CAVITY) /f/ ADJUSTABLE VOLTAGE 25- mi DEVICE /26 /Q24 *27REGULATED VOLTAGE SOURCE y \I3 FREO. ADJUSTING DEVICE /'0 /30 /40 OUTPUTR.I'. POWER ATTENUATOR ATTENUATOR o O o l GO/ GD/ c RELAY Isa Ieb o I7VOLTAGE ATTENUATION DB m A m I'G 3 E5 55 FREQUENCY- KMC.

May 28, 1963 1 c. EISAMAN ET AL 3,091,741

ATTENUATORS Filed April 18, 1957 5 sheets-sheet s United States Patent O3,091,741 ATTENUATORS Leo Coburn Eisaman and Seymour Schilf, Rochester,NX., assignors to General Dynamics Corporation, Rochester, NSY., acorporation of Delaware Filed Apr. 18, 1957, Ser. No. 653,678 1 Claim.(Cl. 331-183) This invention relates to attenuators and is particularlydirected to attenuators for microwave energy, with special attention tomeans for changing attenuation rapidly from one level to another.

So called ferrite devices, using the Faraday principle of rotation ofelectric lines of force in a paramagnetic body, have been used in waveguides for various purposes. A typical device comprises a disk or rod ofa magnetized metal compound, such as nickel-zinc ferrite, placed in awave guide with a magnetizing coil around and coaxial with the rod. Theplane of polarization of the electric field in the guide is rotatedthrough an angle of up to 90, the amount of rotating being a function ofand dependent upon the magnitude of the coaxial magnetic field.Fortunately, this rotation is effected with substantially no insertionloss. A flat metal vane or blade mounted downstream from the ferriteplug will absorb little of the microwave energy when the plane ofpolarization is norrnal to the vane but `will attenuate a maximum amountwhen parallel to the plane of polarization.

The vane in such an assembly has been mechanically rotated to controlattenuation, but the manufacture of suitable rotating joints in the Waveguide is costly and tbe finished product has many operatingdisadvantages.

An object of this invention is to provide an improved microwaveattenuator, a more specic object being the elimination of the mechanicalrotating wave guide joint. In some microwave equipments, the attenuationmust be done in steps from one accurately predetermined level toanother. In such equipments, it is sometimes desired to make the stepchanges suddenly to produce a substantially square-cornered change inthe microwave energy. Obviously, the mechanical inertia of any movingparts mitigates against fast attenuation changes. Accordingly, a furtherobject of this invention is to provide improved means for changing theattenuation in a microwave system with no time delays.

Further, in the ferrite devices of the type contemplated here, it hasbeen found that attenuation is a function of frequency of the microwavesource and that the device is quite sensitive to changes in frequency.Accordingly, the step-levels of attenuation may be unpredictable withsmall drifts in frequency of the microwave source. Recalibration of theattenuator for such frequency drift is impractical. Hence, a stillfurther object of this invention is to provide improved means for makingthe attenuator insensitive to frequency.

Still further, it has been found that the attenuation for any particularlevel of magnetizing current may be of two different values, dependingon the direction of change of magnetizing current as the current arrivesat the new level. |For example, if the magnetizing current is changedfrom -5 milliamperes to +5 milliamperes the attenuation might be l0 db,whereas if the current is changed from milliamperes to +5 milliamperesthe attenuation could be about 13 db. The explanation of theseinconsistent results is probably found in the hysteresis loops of themagnetic materials involved. Accordingly, a still further object of thisinvention is to providerneans for obviating the hysteresis effects ofthe control current-attenuation characteristics of ferrite-typeattenuators.

The objects of this invention are attained by coupling one or moreferritetype attenuators in a microwave transmission line, such as a waveguide, each of the attenu- 3,091,741 Patented May 28, 1963 ators havinga magnetizing coil and being responsive to the changing magnetizingforce of said coil to change the magnitude of attenuation of themicrowave energy; a switch being connected between each coil and saidsource for selectively adding predetermined amounts or steps ofmagnetizing current, and hence causing steps of attenuation in the highfrequency system. The voltage source for the magnetizing current isregulated and is adjustable in response to frequency changes in themicrowave energy.

`Other objects and features of this invention will become apparent tothose skilled in the art by referring to the specific embodimentsdescribed in the following specification and shown in the accompanyingdrawings in which:

FIGURE la is a longitudinal cross-section of one ferritetype attenuatordevice which can be used in the system of this invention,

FIGURE 1b is an end View of the device of FIGURE la,

FIGURE 2 is a graph of the coil current-attenuation characteristic ofthe device of FIGURE 1,

FIGURE 3 is a circuit diagram of one embodiment of the attenuator systemof this invention,

FIGURE 4 is a graph of the frequency-attenuation characteristic of thesystem of FIGURE 3, and

FIGURE 5 is a circuit diagram of a voltage regulator of this invention.

vThe wave guide 1a of FIGURE 1 is assumed to be connected between asource of microwave energy on the one hand and a load or utilizationmeans on the other so that microwave energy ilows from left to right.The wave guide is preferably round in cross-section, is of goodelectrical conductive metal and is internally polished to reduce losses.Concentrically within the guide is the rod 2 of compressed particles ofmagnetic metal, such as nickelzinc ferrite. The axial dimensions of theplug should be of the order of a Wavelength at the operating frequency.A low loss dielectric material, such as Teflon at 3, supports theferrite rod. Surrounding the wave guide and coaxial therewith is thedirect current magnetizing coil 5 with leads 6 which are connected to adirect current source for establishing an axial magnetic field. Becauseof the Faraday effect rotation, the plane of polarization of electricfields approaching the rod 2 is rotated through a finite angle at theother end and the energy continues down the wave guide with but littleattenuation. The amount of rotation depends upon the magnitude of themagnetic field and, hence, may be controlled by the direct currentapplied to leads 6. Downstream from the rod is mounted the vane 7 ofpoor electrically conductive or lossy metal, mounted centrally in theguide on one diameter thereof. When the plane of polarization of themicrowave energy is perpendicular to the vane 7, absorption of energy inthe vane is small. As the component of energy parallel to the planeincreases, attenuation increases.

lFIGURE 2 shows the change 4in attenuation of such a device as themagnetizing coil current changes. It is to be noted that the attenuationis different for an increasing coil current than for a decreasing coilcurrent, and that the characteristic curves are analogous tov a magnetichysteresis loop. 'Ibis feature will be referred to in greater detailhereinafter. -It will be noted further that the curvature of thecharacteristic Varies considerably and that minimum attenuation (nearlyzero) in one particular attenuator occurs when the coil current is about30 ma. in one direction, which is referenced as negative in thisexample. Attenuations of 5 or more db lis obtained when the coil currentis reversed and returned to zero.

In FIGURE 3 the attenuator 10, of the type of FIG- URE 1, is shown witha microwave source 1 at one end and a microwave output on the other. Thetwo leads 6 of the magnetizing coil are connected through switches 11and 12, to the regulated direct current voltage source 13, through theadjustable voltage regulator 14. The voltage output of regulator 14, inFIGURE 3, is stable at any level, but is adjustable to different levels.Adjustment is interlocked or ganged with the frequency determiningdevice at the microwave source 1 so that voltage applied to theattenuator coils is a function of microwave frequency. The interlockwill be referred to more in detail below.

In the embodiment shown in FIGURE 3, it is contemplated that twopredetermined values of coil current be selectively applied to theattenuator 10 to provide two accurately predetermined levels ofattenuation. Nhen an attenuator is used having the characteristic ofFIGURE 2, a reversible coil current is indicated. Conveniently, theswitches 11 and l2 .are connected to reverse the coil current and areoperated by the relay winding 15, which isv in turn energized throughswitch 16 and the relay voltage source at 17. The magnetizing circuitshown has one side grounded, as at 18 and 19. The amplitude of themagnetizing current is adjustable separately for either polarity bypotentiometers 2) and 2l, each adjustment being made to establish thetwo desired levels of attenuation in attenuator 1t). Obviously, theswitching and potentiometer arrangement shown can be easily modified toprovide .any two attenuation levels. For example, instead of switchingfrom minus to plus polarity or vice versa, switching may be made betweentwo plus levels, between two minus levels, or between zero and plus orminus.

Switch 16 for operating the relay may be operated manually orautomatically, as by cams. Further, it is contemplated that the relaysshown, including switches 11 and l2, may be replaced with any desiredsystem of switching, including cam operated microswitches That is, it iswithin the scope of this invention to program the attenuator 10 tooperate between its two levels of attenuation according to anypredetermined plan, such as .a punched-tape program.

By connecting two, three, or more attenuators in the transmissionsystem, the diversity of the program may be increased. For example,attenuators 3@ and 4G may be connected electrically in series with thehigh frequency circuit of attenuator l0. By connecting their coil leads6a and 6b through switches 11a, 12a, llb, .and 12b, the threeattenuators may be programmed to attenuate the wave guide energy to anyof a large number of levels. Relays 15a and 15b are actuated by switcheslea and leb, respectively, to control attenuators 30 and 4o. If, forexample, attenuators 1t?, 30 and 40 are, respectively, adjusted to eachattenuate zero to 10, zero to 20, and zero to 30, any attenuation up to60 db may be obtained, in 10 db steps by manipulating switches 16, 16a,.and lob.

As mentioned above in connection with FIGURE 2, two inconsistent valuesof attenuation can be obtained from a single setting of attenuator coilcurrent. According to an important feature of this invention, it iscontemplated that the controls for the coil current be so constructedand arranged that the level of coil current for a desired level ofattenuation will always be approached from one direction. `In theembodiment shown, in which only two values of coil current are employed,either one of the values may be the reference for the other. By acertain rotational placement of the vane 7 with respect to the normalpolarization plane, one value of coil current, for example, may behighly negative to reduce attenuation to near zero, and the other valuemay be at some less negative or positive potential corresponding to apredetermined attenuation value. Let it be assumed that the secondselected coil current is to be arrived at only by increasing the coilcurrent in a positive direction. That is, potentiometer 20 couldnormally be held in circuit when relay coil 15 is de-energized, .andpotentiometer 20 could be adjusted for the necessary high negative Coilcurrent and its corresponding minimum attenuation. Upon energization ofthe relay, the current direction is reversed in leads 6 andpotentiometer 2l is brought into the circuit which may now be adjustedfor desired ,attenuation, say l0 db. Hence, upon the opening of controlswitch 16, the relay relaxes, the armature drops back, and theattenuator is automatically returned to nearly Zero insertion loss. Itfollows that each time switch 16 is closed the coil current throughpotentiometer 2l `always increases in a positive direction to the presetcoil current and attenuation level.

Although a two-position reversing switch is shown for each attenuator,it is contemplated that three or more potentials could be applied to theattenuator coil. Such a modification would merely require multiplicationof the switching circuits Ll ,and 12 and the provision of the necessaryadditional potentiometers corresponding to 20 and 21 to provide theadditional attenuation levels.

It is further contemplated that the attenuators in the high frequencytransmission system may be placed in various parallel .andseries-parallel combinations, as well as the simple series arrangementshown.

The attenuator of the fcrritetype shown here is quite frequencysensitive. Curve A in FIGURE 4, for example, shows that attenuation ofthe commercially available attenuator of the type known .as theGyraline, No. R- 1350, varies between 18 yand 4 db over a frequencyrange of 8.9 kilomegacycles to 9.6 kilomegacycles per second. Accordingto a still further feature of this invention, means is provided forrendering the insertion loss substantially independent of frequency overthe operating range. The characteristic of the attenuator, when operatedaccording to this invention, is shown by curve B.

It is contemplated that the oscillator-source 1 be adjustable over arelatively wide range of frequencies, such as 8 to 10 kilomegacycles.From curve A of FIGURE 4 it is apparent that considerable coil voltageadjustments must be made as the frequency of the source changes. If theoscillator is or the microwave type which can be tuned by the resonantcavity 22, the knob 23 can be employed for changing the size of thecavity and for changiner the frequency of oscillation. According to animportant feature of this invention, the frequency adjusting device 23is mechanically interlocked, through linkage 24, with the adjustablevoltage device 14 so that the regulated voltage on the leads 19-25becomes a function of the frequency of the system.

One voltage adjusting device 14 is shown, by way of example, in detailin FIGURE 5. To the leads 26 and 2,7, from the regulated source 1.3, ispreferably applied two regulated voltages, on either side of ground.Across the leads 26 and 27 is connected the potentiometer comprisingresistors 28, 2.9, and the anode-cathode space of double triode 31. Asthe grid voltage changes the cathode end of resistor 29 can be smoothlyvaried from positive to negative values including ground potential. Thegrids .are connected in parallel to the output of the second twincathode-follower 32, one grid 33 of which is connected to the adjustablebiasing potentiometer 34 in the cathode circuit as shown. The adjustablecontact of the potentiometer is mechanically linked to the frequencyadjusting device 23 through linkage 24. It is relatively simple toselect the gain of the cathode followers and calibrate the output D.C.voltage at 19-25 in terms of shaft position at 23 and, hence, in termsof microwave frequency. Other circuits could, of course, be designed foradjusting attenuator controlling current in terms 0f oscillatorfrequency.

Many modifications may be made in the relay circuits of FIGURE 3, or theVoltage regulator circuits of FIG- URE 5, or the mechanical details ofthe attenuator of FIGURE 1 without departing from the scope of thisinvention as defined in the following claim.

We claim:

The combination comprising a microwave source with a frequency adjustingdevice, an attenuator for high frequency connected in the output of saidsource, said attenuator being responsive to .a variable direct currentto change the level of lattenuation; a direct current voltage sourceconnected to said attenuator, means for adjusting the voltage of saiddirect current voltage source; and common control means for changingsaid frequency adjusting device and said direct current voltage source,the incremental change in direct current per unit of frequency changebeing so selected as to produce uniform attenuation over a predeterminedrange of frequencies.

2,355,338 Stewart Aug. 8, 1944 6 Haeff et a1 Mar. 18, Zalesk Mar. 9,Norton Aug. 2, Hogan May 29, Reggia July 2, Lee Feb, 25, Zaleski Oct.21, Fox Feb. 24,

OTHER REFERENCES Fox et al.: Behavior and Applications of Ferrites inthe Microwave Region, the Bell System Technical Journal, vol. 34, Numberl, January 1955, pages 1-104 (pages 2633 especially relied upon).

